ELECTRONIC BAND-STRUCTURE STUDY OF THE ANOMALOUS ELECTRICAL AND SUPERCONDUCTING PROPERTIES OF HEXAGONAL ALKALI TUNGSTEN BRONZES A(X)WO(3) (A=K, RB, CS)

The electrical and superconducting properties of hexagonal alkali tung sten bronzes A(x)WO(3) (A = K, Rb, Cs) were examined by calculating th e electronic band structure of a representative hexagonal tungsten bro nze and analyzing reported crystal structures of A(x)WO(3) (A = K, Rb, Cs). These bronzes possess one-dimensional (1D) and three-dimensional Fermi surfaces. The metal-to-semiconductor-to-metal transitions and s uperlattice reflections in KxWO3 and RbxWO3 are explained by a charge density wave (CDW) associated with the 1D Fermi surface. There occurs a maximum in the plots of the CDW onset temperature T-B versus x for K xWO3 and RbxWO3. The presence of this maximum and the absence of a CDW in CsxWO3 reflect the balance of two opposing energy factors, the ele ctronic instability and lattice stiffness, in forming a CDW. The depen dence of the superconducting transition temperature T-c on x suggests that a CDW transition removes lattice phonons conducive for supercondu ctivity.